Halation refers to the spreading of light around bright areas in an image, typically a photograph. The term is also sometimes used in the context of movies and television. As a result of halation, the overall image has a blurred, ethereal quality. The phenomenon can very irksome to photographers when it is not desired, although some deliberately attempt to achieve it for a particular look. A number of things can be used to create accidental or intentional halation.
When halation is accidental and undesired, it usually happens when the photograph is taken. As the light-sensitive emulsion on photographic film is exposed, the light passes through the emulsion and then bounces back, creating a blurred halo around areas of particular brightness. Many film companies combat this by including an antihalation later in their film, to prevent this light scatter. When the film is processed, this layer is washed out so that the film will develop normally.
Some amateur photographers notice problems with halation when they have their film developed as a quickie lab, or when they use disposable cameras. Despite the temptation to blame the staff of the lab for the problem, the halation is actually caused by the film, and not by the staff. Other problems such as scratches or fogging of the film can safely be blamed on a photo lab, as these are indicators that the film was poorly handled as it was processed.
When a photographer wants to deliberately create halation, it is often done through tricks of lighting. It can also be accomplished in the darkroom, through particular handling of the film and developing paper. When halation is actually desired, the photographer can control the effect, creating a softened, blurred look which is often popular for photographs. Experimental photographers also play with halation. The deliberate introduction of halation to an image can often create quite a striking a memorable scene.
A film package will usually indicate whether or not the film has an antihalation layer, in the area with the film's general technical specifications. Photographers who want to explore the phenomenon can purchase film without this protective layer, although they may want to experiment with several different brands and compare them. Many film manufacturers make lines of film with and without this layer, since there is a popular demand for both styles. Photographers who have been struggling with halation, however, should obviously choose film which does include this layer.